Neogene sapropels in the Mediterranean: a review

For 50 years the existence of sapropels (organic-carbon-rich sediments) deposited within Plio–Pleistocene sediments of the Mediterranean Sea has been known. Initially, research concentrated on material recovered in relatively short gravity/piston cores taken from the eastern basins where sequences were found to be well developed/preserved and had extensive spatial coverage. In the main, previous studies concentrated upon establishing a workable stratigraphy, spatial correlation of individual layers and determining the probable depositional mechanisms. However, despite a plethora of research papers, some issues still remain unresolved. This is in part due to a lack of agreement between investigators; sampling and analytical short comings, restricted sample size and the fact that, in many instances, like was not being compared with like. Recently, the limit of sapropels in the western basin has been further extended. As a result, the palaeoceanographic/palaeoclimate models which had previously been developed for deposition of sapropels in the eastern basin have been modified. Most recently, strong links have been established between astronomical cyclicity and sapropel formation. This review paper provides a summary of sapropel research to date, and ongoing sapropel research in the Mediterranean, some of which appears in this thematic issue of Marine Geology. It is fitting that this thematic issue of Marine Geology be dedicated to the memory of Colette Vergnaud-Grazzini and Rob Kidd who in many ways helped to initiate the resurgence in sapropel studies in the 1970s in the Mediterranean —perhaps in 50 more years we will know all of the answers!     

Eastern Mediterranean palaeoclimates from 26 to 5 ka B.P. documented by pollen and isotopic analysis of a core in the anoxic Bannock Basin

Continuous pollen and isotopic records were established for core BAN 84 09 GC retrieved from the anoxic Bannock Basin in the Eastern Mediterranean. On the basis of two ¹⁴C dates, they document the palaeoclimate between about 25.7 ka B.P. and 5.2 ka B.P. in the northern borderlands of the Ionian Basin. The upper half of the core has been redeposited.

The isotopic record displays a correlation with pollen percentages that is strong and positive for Artemisia (sage-brush) and negative for Quercus (oak). The last glacial maximum and the deglaciation are identified by these combined taxa, together with Chenopodiaceae. The glacial maximum around 18 ka B.P. (which has elsewhere been dated from 20 to 15 ka B.P.) has pollen percentages that are high for Artemisia and low for Quercus. The climate in the pollen source area was arid, cold in winter, briefly warm in summer and sustained the vegetation of a semi-desert. The onset of deglaciation after 18 ka B.P. coincides with that of the decline in Artemisia pollen percentage. However, this decline does not indicate reduced aridity, because it is accompanied by a pollen percentage rise of the even more arid herbs Chenopodiaceae and Ephedra. Throughout the deglaciation from 18 to 11 ka B.P., the aridity progressively increases, culminating at 11 ka B.P. This trend is briefly interrupted by a more humid event, shown by a peak in Artemisia pollen percentage and a smaller peak in oak; these two peaks are coeval with the Bölling-Alleröd chronozone (13-11 ka B.P.). Maximum aridity occurs during the Younger Dryas chronozone (11-10 ka B.P.). Afterwards, the oak pollen percentage begins a steady increase, and its maximum value is coeval with the lowest isotopic value, dated at 8760 ± 170 yr B.P. This period was one of high moisture, warm summers, and, according to altitude, mild to cool winters. This climate sustained forests that were Mediterranean in the lowlands and warm temperate in the uplands. A high pollen concentration is observed during this period and reveals the presence of sapropel S1, which is otherwise unrecognizable in this entirely black core. During the following period between 8760 ± 170 and 5200 yr B.P., the δ¹⁸0 reverts to slightly higher values and the Quercus pollen percentage decreases, while the pollen percentage of the wetter Ostrya, the oriental hornbeam, increases. The high pollen concentration during the deposition of sapropel S1 cannot have been caused by increased pollen input into the sea, this pollen being wind-borne, nor by increased pollen production for all taxa, both trees and herbs. We conclude that it is entirely due to increased preservation of this allochtonous organic material by the deep anoxia of the bottom water, below a thick anoxic water column. The coincidence of sapropel deposition with warm and humid local climate as well as with the second global meltwater pulse suggests that the cessation of bottom-water ventilation was due to decreased surface water density, resulting from less saline incoming Atlantic surface water, increased local runoff, and warmer winters.     

Diagenetic magnetic enhancement of sapropels from the eastern Mediterranean Sea

Diagenetic dissolution of magnetic minerals has been widely observed in organic-rich sediments from many environments. Organic-rich sediments from the eastern Mediterranean Sea (sapropels), recovered during Leg 160 of the Ocean Drilling Program, reveal a surprising catalogue of magnetic properties. Sapropels, from all sites studied across the eastern Mediterranean Sea, are strongly magnetic and the magnetization is directly proportional to the organic carbon content. The magnetization of the sapropels is dominated by a low-coercivity, probably single domain magnetic mineral (with an inverse magnetic fabric) that exhibits a clear decay in magnetic properties when exposed to air. During heating, the magnetic particles irreversibly break down between 360 and 400°C. The contrast between the magnetic properties of sapropels and surrounding sediments is marked, with remanence intensities of sapropels often being more than three orders of magnitude higher than those of underlying sediments. The contrast between the magnetic properties of sapropels and the surrounding sediments is apparently controlled by non-steady-state diagenesis: sulphate-reducing conditions dominated during sapropel deposition, while overlying sediments were deposited under oxic conditions. The mineral responsible for the magnetic properties of sapropels is most likely to have formed under sulphate-reducing conditions that existed during times of sapropel formation. Attempts to identify this mineral have been unsuccessful, but several lines of evidence point toward an unknown ferrimagnetic iron sulphide phase. The influence of diagenesis on the magnetic properties of cyclically-deposited eastern Mediterranean sedimentary sequences suggests that magnetic parameters may be a useful proxy for diagenesis in these sediments.     

Carbon and nitrogen isotope excursions in mid-Pleistocene sapropels from the Tyrrhenian Basin: Evidence for climate-induced increases in microbial primary production

The modern Mediterranean Sea is oligotrophic, yet its sediment record contains layers of organic-carbon-rich sapropels at 21 ky (precessional) spacing that imply periods of elevated paleoproductivity that approached the high productivities of modern upwelling systems. Resolution to this paradox is provided by lines of evidence suggesting that the mode of primary productivity changed from one dominated by algae to one during times of sapropel deposition in which photosynthetic bacteria were important. We have made a high-resolution comparison of the organic carbon and nitrogen isotopic compositions of three sapropels and their background sediments in a 3-m sequence that corresponds to 1001 to 946 ka. Organic δ¹³C values systematically increase from − 26‰ to − 21‰ and δ¹⁵N values systematically decrease from 4‰ to < 0‰ as organic carbon mass accumulation rates increase in the sapropel layers. The increase in carbon isotope values mirrors the increases in primary productivity and associated organic matter export indicated by the increased mass accumulation rates. The decrease in nitrogen isotope values implies major contributions of nitrogen-fixing cyanobacteria to the total marine productivity. The precessional minima with which sapropels coincide were times of wetter climate that stratified the surface Mediterranean Sea, increased delivery of soil-derived phosphorus, and evidently amplified microbial primary production. Our high-resolution study reveals several relatively rapid excursions into and out of the high-productivity mode that suggest that sapropel deposition was a climate-sensitive surface-driven phenomenon that was not accompanied by basin-wide stagnation.     

The occurrence and significance of Pleistocene and Upper Pliocene sapropels in the Tyrrhenian Sea

Numerous sapropels and sapropelic strata from Upper Pliocene and Pleistocene hemipelagic sediments of the Tyrrhenian Sea show that intermittent anoxia, possibly related to strongly increased biological productivity, was not restricted to the eastern Mediterranean basins and may be a basin-wide result of Late Pliocene-Pleistocene climatic variability. Even though the sapropel assemblage of the Tyrrhenian Sea clearly originates from multiple processes such as deposition under anoxic conditions or during spikes in surface water productivity and lateral transport of organic-rich suspensates, many “pelagic sapropels” have been recognized. Stratigraphic ages calculated for the organic-rich strata recovered during ODP Leg 107 indicate that the frequency of sapropel formation increased from the lowermost Pleistocene to the base of the Jaramillo magnetic event, coinciding with a period when stable isotope records of planktonic foraminifera indicate the onset of climatic cooling in the Mediterranean. A second, very pronounced peak in sapropel formation occurred in the Middle to Late Pleistocene (0.73-0.26 Ma). Formainifers studied in three high-resolution sample sets suggest that changes in surface-water temperature may have been responsible for establishing anoxic conditions, while salinity differences were not noted in the faunal assemblage. However, comparison of sapropel occurrence at Site 653 with the oxygen isotopic record of planktonic foraminifers established by Thunell et al. (Proc. ODP, Sci. Results 107, 1990) indicates that sapropel occurrences coincide with negative δ¹⁸O excursions in planktonic foraminifers in thirteen of eighteen sapropels recognized in Hole 653A. A variant of the meltwater hypothesis accepted for sapropel formation in the Late Pleistocene eastern Mediterranean may thus be the cause of several “anoxic events” in the Tyrrhenian as well. Model calculations indicate that the amount of oxygen advection from Western Mediterranean Deep Water exerts the dominant control on the oxygen content in deep water of the Tyrrhenian Sea. Inhibition of deep-water formation in the northern Adriatic and the Balearic Basin by increased meltwater discharge and changing storm patterns during climatic amelioration may thus be responsible for sapropel formation in the Tyrrhenian Sea.     

Messinian pre-evaporite sapropels and precession-induced oscillations in western Mediterranean climate

Cyclical fluctuations in planktic foraminiferal assemblages have been recognized in the pre-evaporitic Messinian in a marginal basin of the western Mediterranean. The fluctuations coincide with a dominantly precession-controlled sedimentary cyclicity (sapropels). During sapropel deposition, high planktic foraminiferal diversities are indicative of relatively stable marine conditions, while during homogeneous marl deposition low diversities seem to indicate the presence of unfavourable, more saline surface water conditions. The dominance of a precession-related signal indicates that regional climate oscillations rather than (obliquity-related) glacio-eustatically controlled influxes of Atlantic and/or Mediterranean waters are responsible for the faunal fluctuations and sedimentary cyclicity. Our scenario links the persistence of normal marine conditions during sapropel formation with increased rainfall and run-off along the western Mediterranean at times that perihelion occurred in Northern Hemisphere summer. Less favourable, highly saline surface water conditions prevailed during periods of drier climate induced by opposite precessional extremes. The cyclical oceanographic fluctuations could also have governed periodic reef growth along the margins.     

Benthic foraminiferal successions across Late Quaternary Mediterranean sapropels

Benthic foraminiferal records across Late Quaternary Mediterranean sapropels S6, S5 and S1 are reviewed and re-considered in the light of recent advances in foraminiferal ecology. It is suggested that the main factor controlling the foraminiferal successions that occur immediately before and after sapropel deposition is the amount of time involved in the onset of anoxic conditions and in the re-oxygenation of the benthic environment. Faunas dominated by deep infaunal taxa such as Globobulimina and Chilostomella reflect a very gradual decrease or increase of bottom water oxygenation. Post-sapropel faunas dominated by small biconvex, trochospiral taxa, which are inferred to have a much more opportunistic life strategy, are typical of a very rapid re-oxygenation of the benthic environment after sapropel deposition. The time involved in re-oxygenation depends upon the mechanism causing sapropel formation. The faunal succession found above sapropel S5 suggests a basin with a strong density gradient in the upper part of the water column during sapropel deposition, followed by a rapid turnover of the following column and quick re-oxygenation of the benthic environment. In contrast, the faunal succession above sapropel S6 suggests a water column with a more gradual density gradient deeper in the basin, a progressive deepening of the halocline, and a very slow increase of bottom water oxygenation.     

Middle Miocene (Langhian) sapropel formation in the easternmost Mediterranean deep-water basin: Evidence from northern Cyprus

Mid-Miocene (Langhian; ∼15.4 Ma) sapropels formed within the easternmost Mediterranean basin, now uplifted in northern Cyprus. These sapropels represent the oldest known sapropels in a predominantly marl succession. Six well-developed sapropels were studied. Strontium isotope dating of twelve samples gave a preferred age of ∼15.4 Ma (Langhian); i.e. during the final phases of the Middle Miocene Climatic Optimum (MCO). The age of the best-preserved nannofossil assemblage (Langhian) is close to the strontium ages. The Langhian strontium ages are preferred over an alternative early Serravallian age for less well-preserved nannofossil assemblages. Total organic carbon contents in the sapropels reach maximum values of 3.9 wt.%. Relative to the host marls, the sapropels show enrichments in terrigenous-derived minerals and related major and trace elements. Sedimentological evidence indicates that the terrigenous sediments were eroded from the northern borderlands of the deep-water basin under warm, humid conditions. High fresh-water run-off from surrounding landmasses is likely to have promoted a low-salinity lid to the eastern Mediterranean deep-water basin. This, in turn, would have restricted deep-water ventilation and promoted widespread anoxia. Exceptionally high concentrations of chalcophile elements (e.g. Cu, Ni and Zn) are consistent with anoxic conditions. Abundant nutrient-rich fresh-water input is also likely to have stimulated siliceous productivity (although any siliceous microfossils did not survive diagenesis). A significant role for diagenesis in sapropel formation is indicated by the mobilisation of Ba from sapropels to marl directly beneath. Orbitally induced dry–wet oscillation, the mechanism invoked to explain the Pliocene to Holocene sapropels, apparently was already in place during the latest stages of the MCO when the Langhian sapropels accumulated. These sapropels accumulated immediately after the Middle Miocene closure of the Southern Neotethys when the Eastern Mediterranean Sea apparently became more sensitive to orbital cyclicity. The development of a semi-enclosed deep-water basin was, therefore, a prerequisite for sapropel formation.     

Origin and transformation of organic matter in Pliocene–Pleistocene Mediterranean sapropels: organic geochemical evidence reviewed

Drilling of deep-water post-Messinian sedimentary sequences by ODP Legs 160 and 161 in the Mediterranean Sea has shown that occurrence of organic-carbon-rich sapropels and sapropel-like sediments extends from the Levantine Basin westward into the Alboran Basin. In the eastern Mediterranean, sapropel deposition started in the Early Pliocene, whereas in the Western Basin the onset of sapropel formation occurred later, in the Early Pleistocene. Precessional cycles are apparently the primary external forcing for sapropel formation. Nevertheless, the pattern of sapropel occurrence suggests that the precessional influence is modulated by the glaciation cycles. Large differences were observed in the organic carbon contents of sapropels recovered in the eastern and western Mediterranean. Correspondence between organic carbon contents, Rock-Eval hydrogen index values and elemental C/N ratios indicate that both variations in the production and preservation of marine organic matter have led to the accumulation of high amounts of organic matter in sapropels. Molecular organic geochemical compositions of sapropels from the eastern Mediterranean further confirm that the major fraction of organic matter in sapropels is derived from marine algal sources and has undergone variable oxidation. Enhanced marine productivity and improved preservation of organic matter is central to sapropel formation. Accumulation of increased amounts of land-derived material at times of sapropel formation is also evidenced, supporting the hypothesis of significant periodic freshwater discharges.     

Calcareous nannofossil and planktonic foraminiferal distributional patterns during deposition of sapropels S6, S5 and S1 in the Libyan Sea (Eastern Mediterranean)

In core ADE3-23 collected in the Libyan Sea, the nannofossil species Coccolithus pelagicus, Coronosphaera spp., Helicosphaera spp., Syracosphaera spp., Calcidiscus spp., small Gephyrocapsa spp., and the planktonic foraminifers Globigerina bulloides, Neogloboquadrina pachyderma, Globorotalia scitula, Turborotalita quinqueloba and Neogloboquadrina dutertrei prevail in sapropel S6 (midpoint at 172 ka b.p.), indicative of cold and highly productive surface conditions. Warm and highly stratified water-column conditions are recorded by the characteristic assemblage of Globigerinoides ruber, Globoturborotalita rubescens, Florisphaera profunda, Rhabdosphaera spp. during the sapropel S5 depositional interval (midpoint at 124 ka b.p.). Compared with S5, Globigerinita glutinata, Globorotalia inflata, Globigerinella siphonifera, Globorotalia truncatulinoides and the calcareous nannofossil Emiliania huxleyi characterise less stratified conditions within sapropel S1 (midpoint at 8.5 ka b.p.). Multivariate statistical analyses of calcareous nannofossil and planktonic foraminifers in core ADE3-23 identify planktonic assemblages which typify sapropels S6, S5 and S1 in the Libyan Sea. A warmer interval is recognised in the middle part of the cold S6, and can be associated with an influx of less saline waters and the occurrence of a faint, temporary deep chlorophyll maximum. Evidence for enhanced surface productivity and breakdown of stratification is observed in the middle–upper part of the warm S5, associated with climatic deterioration. Moreover, an increase in surface productivity in the upper S1 implies weak stratification. Our combined calcareous nannofossil and planktonic foraminiferal data add to the evidence that climate variability was more pronounced than commonly considered to date for all the three studied Eastern Mediterranean sapropel depositional intervals.     

Mediterranean functioning and sapropel formation: respective influences of climate and hydrological changes in the Atlantic and the Mediterranean

During Ocean Drilling Program (ODP) Legs 160 and 161, sapropels were recovered both in the western and eastern Mediterranean. This obliges to a reassessment of the previous studies focused on sapropels from only the eastern Mediterranean, and to consider the changes which occurred in the Mediterranean climate but also in the water characteristics both in the Atlantic and in the western Mediterranean. In the North Atlantic, the position of the polar front which migrated southwards during glacial times and the melting of northern ice caps during interglacial periods, together with the convection in the Labrador and Norwegian Seas, appear essential to control the salinities of the waters facing the Strait of Gibraltar. The salinities of the surface and intermediate layers constitute the first driving force of the Mediterranean dynamics, the second driving force being the Mediterranean climate. The stagnation of deep waters leading to sapropel deposition in the western Mediterranean may be explained by a drastic weakening of the density difference between Mediterranean outflow and Atlantic intermediate waters facing the Strait of Gibraltar. This weakening was induced primarily by the salinity decrease of Atlantic surface water and secondly by a rather high salinity in the Atlantic intermediate layer, rather than by a drastic deterioration of the Mediterranean climate. This scenario probably concerns most of the sapropel events and it may be used for the knowledge of Atlantic and Mediterranean functioning over climatic changes.     

Sedimentology and geochemistry of an exceptionally preserved last interglacial sapropel S5 in the Levantine Basin (Mediterranean Sea)

A combined study of lithological, geochemical and physical sediment properties is reported from a completely laminated S5 sapropel, recovered in three gravity cores (M40-4 SL67, M51-3 SL103, M51-3 SL104) from the Pliny Trench region of the eastern Mediterranean. The thickness of the studied sapropel S5 varies between 85 and 91 cm and tops most S5-sapropels in the Mediterranean. Based on optical features like color and thickness of laminae, the sapropels were subdivided into thirteen distinct lithostratigraphic zones. These zones, as well as the finer layering pattern within them, could be followed exactly among the three cores, indicating that the processes responsible for this variation acted at least on a regional scale. The sapropel sediment is characterized by exceptionally high porosity, which is strongly correlated with Si/Ca. This relationship implies that the sapropel is in essence an organic-matter rich diatomite and its exceptional thickness can be explained by preservation of diatoms forming a loosely packed sediment fabric. Compared to other S5 sapropels, the preservation of diatoms has apparently led to a twofold increase in the thickness of the sapropel layer. Relative abundances of 10 elements were determined at ultra-high resolution (0.2 mm) by XRF-scanner over the complete length of each sapropel including several cm of enclosing marl. An analysis of the chemical data indicates that the lowermost 13 cm of the sapropel is chemically more similar to the underlying marl and that the sediment chemistry shows different signals at different scales. The strongest pattern is the contrast between the sapropel and the surrounding marl, which is accentuated in elements indicative for redox conditions as well as terrigenous sediment input and productivity. Within the sapropel, a mm- to cm-scale layering is observed. The abundances of many elements are systematically linked to the pattern of these layers, indicating a common origin, related to productivity and/or terrigenous sediment and/or redox conditions. This pattern indicates a link to a regional climatic process, making the S5 sapropel horizon in M40-4 SL67, M51-3 SL103 and M51-3 SL104 a potential high-resolution archive of climatic variability during the last interglacial in the Mediterranean Sea and its adjacent landmasses.     

Origin and differentiation of clay minerals in pelagic sediments and sapropels of the Bannock Basin (Eastern Mediterranean)

Two cores from an anoxic basin of the southeastern Mediterranean Ridge were investigated to compare the clay mineralogy of pelagic sediments and of the interbedded sapropels. The sediments of Core BAN 84-02, raised from the basin floor, and those of Core BAN 84-08, from the eastern plateau of the Bannock Basin, provide evidence for different sedimentary environments. The anoxic conditions, which are still present near the bottom, produce an important decrease in smectite crystallinity (Core 02), whereas well-organized smectite persists in the normally oxygenated sediments (Core 08). Detrital clay minerals from various sources were deposited in the basin and no appreciable diagenesis was recognized downcore.

The clay mineralogy of the sapropels shows remarkable differences compared to the pelagic sediments. The changes observed are dependent on aggressive chemical reaction and on the sudden input of detrital crystalline sediments into the stagnant environment. A climatic curve registers the variable degree of clay mineral hydrolysis in continental areas and exhibits good correspondence with an already published oxygen isotope curve for the area.     

Modes of sapropel formation in the eastern Mediterranean: some constraints based on pyrite properties

Pyrite formation within and directly below sapropels in the eastern Mediterranean was governed by the relative rates of sulphide production and Fe liberation and supply to the organic-rich layers. At times of relatively high SO²⁻₄ reduction, sulphide could diffuse downward from the sapropel and formed pyrite in underlying sediments. The sources of Fe for pyrite formation comprised detrital Fe and diagenetically liberated Fe(II) from sapropel-underlying sediments. In organic-rich sapropels, input of Fe from the water column via Fe sulphide formation in the water may have been important as well. Rapid pyrite formation at high saturation levels resulted in the formation of framboidal pyrite within the sapropels, whereas below the sapropels slow euhedral pyrite formation at low saturation levels occurred. δ³⁴S values of pyrite are −33‰ to −50‰. Below the sapropels δ³⁴S is lower than within the sapropels, as a result of increased sulphide re-oxidation at times of relatively high sulphide production and concentration when sulphide could escape from the sediment. The percentage of initially formed sulphide that was re-oxidized was estimated from organic carbon fluxes and burial efficiencies in the sediment. It ranges from 34% to 80%, varying significantly between sapropels. Increased palaeoproductivity as well as enhanced preservation contributed to magnified accumulation of organic matter in sapropels.     

Eastern Mediterranean sapropel S1 interruption: an expression of the onset of climatic deterioration around 7 ka BP

We discuss the palaeoclimatic interpretation of unprecedented high-resolution micropalaeontological studies of short-term (2 to 4 centuries) interruptions within early Holocene organic-rich layer (sapropel) S1 from the eastern Mediterranean. Results for cores from the Adriatic and Aegean seas that contain `double' S1 sapropels indicate that these interruptions, which are centred roughly around 7000 years <sup>14</sup>C<sub>nc</sub> BP, are genuine and related to climatic deterioration. This interpretation is endorsed by a coeval dry event recorded in terrestrial records and indications of climatic deterioration affecting human migration patterns and early societies in Egypt. The presence of sapropel interruptions in the two major source areas of deep water for the entire eastern Mediterranean likely implies that similar intervals may be found throughout the basin, provided that sedimentation rates and sampling resolutions allow the detection of events with a duration of only several centuries. Moreover, our results show that the `sapropel mode' of circulation comprises a delicate balance between reduced ventilation and enhanced productivity, which is easily disturbed through surface water cooling triggering a short time of improved deep water ventilation.     

The Messinian event in the Paratethys: Astronomical tuning of the Black Sea Pontian

This study presents new data on the orbitally calibrated Maeotian/Pontian and Pontian record of the Black Sea Basin (Paratethys) obtained by time-series analysis of magnetic susceptibility (MS) data from relatively deep-water Upper Miocene sediments exposed in the Zheleznyi Rog section (Taman Peninsula, Russia). In the studied interval, a ∼145-m-long sedimentary sequence, spectral analysis revealed statistically significant signals with 6.1–8.2 m and 3.0–4.0 m wavelength. These signals correspond to the obliquity and precession cycles, respectively. This study correlates the main steps of Messinian Salinity Crisis (MSC) of the Mediterranean to the Black Sea Pontian record based on astronomical tuning of the study sequence and evaluation of integrated biostratigraphic, paleomagnetic and sedimentological data. Based on cyclostratigraphic results, Maeotian/Pontian beds with Actinocyclus octonarius accumulated from ∼6.3 to 6.1 Ma. Most of the Novorossian sediments correspond to the first MSC step. The TG 22 (5.79 Ma) and TG 20 (5.75 Ma) glacial events occur in the uppermost Novorossian record and are marked by extraordinary high values of MS. The Portaferian, dated at the base as ∼5.65 Ma and the top as ∼5.45 Ma, corresponds to the second MSC step. The Novorossian/Portaferian transition is marked by the hiatus of approximately 150–160 kyr, which agrees well with the concept of the intra-Pontian unconformity in the Black Sea Basin and a sea-level drop in the Mediterranean from 5.6 to 5.46 Ma. The ages for the base and the top of the Bosphorian were estimated as ∼5.45 Ma and ∼5.27 Ma, respectively. The base of the Bosphorian corresponds to the third Lago Mare episode caused by the high sea-level connection between the Mediterranean and Eastern Paratethys.     

Geochemistry of eastern Mediterranean sediments: Primary sediment composition and diagenetic alterations

Two cores recovered in the eastern Mediterranean were analysed for major, minor and trace elements. The primary chemical composition of the sediment is different at each location, probably because the lithological sources and the relative biogenic contributions differ.

Carbonates are important for the concentration of Ca, Mg and Sr, whereas aluminosilicates determine the concentration of Si, Al, K, Li, Y and Be, and to a lesser extent that of Fe, Cr, Ti, Mg, Zn and Zr. In sapropels, organic carbon and sulphur seem to be closely related. Bromine, Mo, P, Fe, V, Cu, Zn, Co, Ni and Cr are closely associated with organic and sulphidic compounds. The concentration versus depth profile for organic carbon in two sapropels points to a rapid establishment of conditions that gave rise to sapropel formation, followed by a gradual transition back to “normal” conditions.

The primary composition is overprinted by diagenetic processes. Sulphate-reducing conditions occurred during and just after sapropel deposition. A progressive oxidation front mechanism, which became active after sapropel deposition, is responsible for additional major geochemical changes. This diagenetic phenomenon has strong implications for the chemistry of Fe, Mn, Ni, Co, Zn, Cu, Cr, V, U, As and Sb.     

Age,regional variation,and shallowest occurrence of S1 sapropel in the northern Aegean Sea

Thirteen gravity cores taken off northern Greece contain the S1 sapropel, as a single layer in cores from water depths <250 m and usually as a double layer in cores from deeper water. Total thickness of the sapropel layer is proportional to thickness of sediment between the sapropel and an 18 ka seismic marker. These observations suggest that the sapropel is not diachronous. Five AMS radiocarbon dates of molluscs and echinoderms indicate an age of 6.4–9.2 ka for the S1 sapropel, considerably younger than previous age estimates in the northern Aegean Sea of 9–13 ka, which were based on bulk sample dating techniques.     

Review of recent advances in the interpretation of eastern Mediterranean sapropel S1 from geochemical evidence

The sediments of the eastern Mediterranean basin contain C_org-enriched layers (sapropels) interbedded with the C_org-poor sediments which form by far the greater part of the record. While it is generally appreciated that different surface ocean productivity and bottom water conditions are necessary for the formation and preservation of these two sediment types, less attention has been paid to diagenetic effects which are an expected consequence of transitions between dramatically different bottom water oxygenation levels. A geochemical interpretation has emerged of post-depositional oxidation of the most recent sapropel (S1), initially based on the relationship of the Mn, Fe, C_org and S concentration/depth profiles observed around S1, and the characteristic shapes of these elemental profiles known from other situations. This indicates that post-depositional oxidation has removed approximately half of the visual evidence of the sapropel (∼6 cm from a total of ∼12 cm in the deep basin). The oxidation interpretation from redox-sensitive element redistribution profiles has subsequently been consolidated with evidence from pore water (O₂, NO⁻₃, Mn²⁺ and Fe²⁺) studies, from characteristic solid phase Ba profiles which yield palaeoproductivity records, and from oxidation-sensitive indicator trace elements (I and Se). So far, these geochemical observations have been concentrated in the deeper central parts of the basin, where sediment accumulation rates are lower than on the basin margins, and radiocarbon dating indicates that S1 formation occurred between 5.3 and 9.0 ky (uncorrected conventional radiocarbon time). It remains to be demonstrated whether or not these times are applicable to the entire E. Mediterranean basin. The implications of these findings to guide sampling in future work on the S1 productivity episode and on older sapropels for palaeoenvironmental investigations are discussed.     

57～48万年前东亚夏季风的神农架石笋记录

应用MC-ICP-MS和MAT-253质谱技术,实测神农架山宝洞一支长达1.2 m石笋(SB32)的3个230Th年龄和236个氧同位素数据。在年龄误差范围内,运用天文轨道调谐方法重建了570~480 kaBP时段百年至千年际分辨率东亚夏季风降水记录。该记录跨越4个完整的岁差旋回,在岁差周期上石笋δ18O平均振幅达4‰。研究时段内东亚夏季风轨道尺度变化仍然受太阳辐射的控制。石笋记录的深海氧同位素阶段MIS14/13转换过程与末次冰消期具有类似特征。一系列千年尺度事件叠加在岁差周期夏季风变化趋势上,表明千年尺度气候变化是地球气候系统内部自我维持的固有频率,可能属于海-陆-气相互作用的结果。